Mono-silane (SiH4), chlorosilane (H3SiCl) and dichlorosilane (H2SiCl2) are useful chemicals for the production of electronic devices based on high purity crystalline silicon. These silicon bearing gases are thermally decomposed to form the high purity silicon material. The production of high purity silane is presently practiced on a commercial scale by a process shown generally in FIG. 1 and generally described by U.S. Pat. No. 4,676,967 wherein, in Zone 1, metallurgical grade silicon (101) is gasified by the reaction of hydrogen and silicon tetrachloride to form a mixture (102) containing volatile trichlorosilane:2H2+3SiCl4+Si→4HSiCl3  (1)Impurities (103) are rejected.
Then, in Zone 2, trichlorosilane is converted to the high purity silane product (201) in a series of distillation separations and catalytic redistribution reactions which also produce silicon tetrachloride (202) as a co-product. Impurities (203) are rejected. The silicon tetrachloride (202) is recycled to Zone 1.4HSiCl3→3SiCl4+SiH4  (2)
The silane (201) is then pyrolyzed in any of several ways in Zone 3 to form ultra-pure silicon (301) and, if the process is close coupled, the by-product hydrogen (302) is recycled to Zone 1.